1. Field of the Invention
The present invention relates to a read-only recording medium on which information has been recorded, and more particularly, to a read-only recording medium with a super-resolution near-field structure (Super-RENS) on which optically readable information has been prerecorded, a method for reading the information, and an apparatus for reproducing the same.
2. Description of the Related Art
Optical discs including digital versatile discs (DVDs) continue to gain popularity as high-density recording media designed to record image data or computer data. In particular, a read-only optical disc such as DVD-ROM on which a movie or computer program has been prerecorded is commonly used to effortlessly distribute massive amounts of information.
Information has been prerecorded on a substrate of a read-only optical disc in the form of marks (pits). In order to read out the information, an optical disc reproducing apparatus emits a laser beam onto the optical disc, and a photodetector detects the intensity of a reflected beam that varies depending on presence or absence of rows of marks. For example, the intensity of the reflected beam is reduced if the marks are present while the intensity is increased if the marks are absent.
Thus, the amount of information that can be recorded on the read-only optical disc is determined by the size of marks (pits) readable in the reproducing apparatus. Reducing the size of marks (pits) increases the density of information that can be recorded on the optical disc by recording more information per disc.
The size of marks readable by the reproducing apparatus is determined by, among other factors, resolution limit (RL) of an optical system of the reproducing apparatus. The RL of the optical system can be theoretically calculated by Equation (1):RL=λ/(4×NA)  (1)where λ is the wavelength of a laser beam and NA is the Numerical Aperture of an objective lens.
In the case of a red laser commonly used, RL of 265 nm is obtained by putting λ=635 nm and NA=0.6 into Equation (1). When a blue laser is used, RL of 156 nm is obtained by putting λ=405 nm and NA=0.65 into Equation (1). That is, an optical disc reproducing apparatus using the red laser does not allow a mark (pit) having a length not exceeding 265 nm to be read. Even in an optical disc reproducing apparatus using a short-wavelength blue laser, it is difficult to read a mark (pit) having a length not exceeding 156 nm.
FIG. 1 is a graph illustrating the relationship between a mark length and a carrier-to-noise ratio (CNR) for a conventional read-only optical disc having only an Ag reflective layer on a substrate. The measurements were made when mark depths were 50, 70, and 100 nm, respectively, and RL of a reproducing apparatus was 265 nm.
As is evident from FIG. 1, it is possible to successfully read information recorded in the form of marks (pits) from the optical disc when a mark length is greater than 290 nm since CNR is greater than 40 dB. However, the CNR is sharply decreased for a mark length less than 290 nm. CNR is about 16 dB for a mark length of 265 nm (i.e., RL of the reproducing apparatus), and if a mark length is less than 250 nm, CNR decreases to about zero.
A Super-RENS has received considerable attention as a technology to improve RL of an optical disc reproducing apparatus as defined by Equation (1), and this structure has been applied to phase-change recording optical discs (See “Applied Physics Letters, Vol.73, No.15, October 1998” and “Japanese Journal of Applied Physics, Vol.39, Part I, No.2B, 2000, pp.980–981”)
In the Super-RENS, a special mask layer is formed on an optical disc, and surface plasmons generated in the mask layer are used to reproduce information. There are two types of Super-RENS: antimony (Sb) transmission and silver oxide (AgOx) decomposition. In the Sb transmission Super-RENS, an Sb mask layer undergoes a phase change due to a laser beam so it becomes transparent. In the AgOx decomposition type Super-RENS, an AgOx mask layer is decomposed into Ag and O by application of a laser beam, and then the Ag generates surface plasmons.
FIG. 2 illustrates the principle of recording on a recordable optical disc using a conventional Super-RENS. As illustrated in FIG. 2, a recording medium has a first dielectric layer 112-1 formed of dielectric material such as ZnS—SiO2 or SiN on a transparent polycarbonate layer 111, a mask layer 113 formed of Sb or AgOx, a protective layer 114 made from dielectric material such as ZnS—SiO2 or SiN, a recording layer 115 formed of GeSbTe, and a second dielectric layer made of dielectric material such as ZnS—SiO2, all of which are sequentially stacked.
Here, the protective layer 114 and the first dielectric layer 112-1 are made from SiN for use with an Sb mask layer 113 while they are made from ZnS—SiO2 for use with an AgOx mask layer 113. The protective layer 114 where near-field interactions occur while reproducing information prevents reactions between the mask layer 113 and the recording layer 115. If the mask layer 113 is made of Sb, the Sb undergoes a phase change upon application of a laser beam so it becomes transparent. If the mask layer 113 is made of AgOx, a laser beam causes AgOx to be decomposed into Ag and O, and the Ag generates local plasmons.
A laser beam is emitted from a laser 117 having an output power of about 10 to 15 mW, and converged onto the recording medium by a converging lens 118. When a region of the recording layer 115 illuminated by laser is heated to a temperature above about 600° C., the region undergoes a phase change to an amorphous state, and an absorption coefficient of the region decreases. At this time, in a region of the mask layer 113 illuminated by laser, the crystal structure of the Sb changes, or AgOx is decomposed in a quasi-reversible reaction. Since the region of the mask layer 113 acts as a probe for the recording layer 115, it is possible to successfully reproduce microscopic marks with a size below RL.
However, unlike the recordable recording medium, a read-only recording medium has marks prefabricated on a substrate as well as a different layer stack structure. Furthermore, the read-only recording medium is required to realize the Super-RENS effect only with application of a weak laser beam of 2–3 mW. Thus, for the read-only recording medium, determining material and the type of layer stack structure that can achieve a high CNR becomes a matter of great concern.